main() {
//1. if语句
  int age = 18;
  if (age >= 18) {
    print("成年");
  }
//2. if-else语句
  if (age >= 18) {
    print("成年");
  } else {
    print("未成年");
  }
//3. if-else-if语句
  int score = 80;
  if (score >= 90) {
    print("优秀");
  } else if (score >= 80) {
    print("良好");
  } else if (score >= 70) {
    print("中等");
  } else if (score >= 60) {
    print("及格");
  } else {
    print("不及格");
  }

  //4. switch语句
  String grade = "A";
  switch(grade){
    case "A":
      print("优秀");
      break;
    case "B":
      print("良好");
      break;
    case "C":
      print("中等");
      break;
    case "D":
      print("及格");
      break;
    default:
      print("不及格");
      break;
  }

  //5. 三元表达式
  String result = age >= 18 ? "成年" : "未成年";
  print(result);
}
